1. Field of the Invention
The present invention relates to a near-infrared to infrared light-receiving element and an optical device and more particularly to a light-receiving element having responsivity that depends moderately on the wavelength in a near-infrared to infrared region and an optical device including the light-receiving element.
2. Description of the Related Art
A near-infrared to infrared region corresponds to an absorption spectrum region relevant to living bodies, such as animals and plants, and the environment. Thus, a near-infrared light detector that contains a III-V group compound semiconductor in a light-receiving layer is under development. For example, Japanese Unexamined Patent Application Publication No. 2010-288297 discloses a planer type light-receiving element that includes an InGaAs/GaAsSb type II multi-quantum well (MQW) structure as a light-receiving layer. This planer type light-receiving element includes a diffusion concentration distribution control layer, which prevents a high concentration of impurities from spreading over a multi-quantum well structure and degrading crystallinity during the formation of a p-n junction or a pin structure in the multi-quantum well structure by selective diffusion.
Although the sensor having an InGaAs/GaAsSb type II multi-quantum well structure as a light-receiving layer has responsivity at a wavelength of up to approximately 2.5 μm, the responsivity fluctuates greatly with the wavelength. This is because the InGaAs/GaAsSb type II multi-quantum well structure undergoes a type I transition as well as a type II transition. The type I transition imparts high responsivity to the sensor at a wavelength in the range of 1 to 1.7 μm. The wavelength in the range of 1 to 1.7 μm is in a type I wavelength region.
In a type I transition resulting from light reception, an electron makes a transition from a valence band to a conduction band in the layer having the multi-quantum well structure. Both the conduction band energy and the valence band energy are higher in GaAsSb than in InGaAs. The energy difference (band gap) Eg between the conduction band energy and the valence band energy is 0.75 eV (λ=1.65 μm) for GaAsSb having a standard chemical composition or 0.74 eV (λ=1.67 μm) for InGaAs having a standard chemical composition. Thus, GaAsSb and InGaAs have substantially the same energy difference (band gap). This defines the upper limit of the type I wavelength region in the range of 1 to 1.7 μm.
In the InGaAs/GaAsSb type II multi-quantum well structure, upon light reception, an electron makes a transition from a layer having a higher valence band energy (GaAsSb layer) to a layer having a lower conduction band energy (InGaAs layer). A small transition energy difference allows responsivity to expand to a longer wavelength. However, since the transition occurs between adjacent layers, the transition probability is surely lower in the type II transition than in the type I transition. In this case, the type II transition probability is proportional to an overlap integral between the wave function of electrons confined in a quantum well in the InGaAs conduction band and the wave function of holes confined in a quantum well in the adjacent GaAsSb valence band. An overlap integral between the adjacent bands is lower than an integral in InGaAs or GaAsSb in the type I transition (an integral in the same layer).
As described above, light reception in the InGaAs/GaAsSb type II multi-quantum well structure causes both the type I transition and the type II transition, and light reception due to the type I transition has higher responsivity than light reception due to the type II transition. This phenomenon is inevitable in the InGaAs/GaAsSb type II multi-quantum well structure. Thus, responsivity at a wavelength in the range of 1 to 1.7 μm is much higher than responsivity at a longer wavelength.
Furthermore, as described above, in a planer type light-receiving element having an InGaAs diffusion concentration distribution control layer, the InGaAs layer receives light having a wavelength in the range of 1 to 1.7 μm. This further increases responsivity in the type I wavelength region as compared with responsivity in a longer-wavelength region. The present invention is not limited to planer type light-receiving elements and is also directed to light-receiving elements having a mesa structure.
The phenomenon in a planer type light-receiving element that includes the InGaAs/GaAsSb type II multi-quantum well structure as a light-receiving layer will be described in detail below. Because of the phenomenon, the short-wavelength region of 1.7 μm or less and the long-wavelength region of more than 1.7 μm have different responsivities. The short-wavelength region of 1.7 μm or less has a mountain-shaped responsivity characteristic with a peak responsivity at a wavelength of approximately 1.5 μm. Responsivity at a wavelength of approximately 1.5 μm is approximately twice and much higher than responsivity in the long-wavelength region at a wavelength of 2 μm.
A sensor including a light-receiving element having such a type II multi-quantum well structure as a light-receiving layer has the following problem. Signals of the sensor are amplified, for example, in an amplifier circuit before output. When responsivity fluctuates greatly in the target wavelength region, the gain must be optimized in accordance with the wavelength region. However, this is practically difficult. From a practical standpoint, a light-receiving element having considerable fluctuations in responsivity cannot be used in a detector for spectroscopic analysis.